This invention relates to a process for producing methyl methacrylate from methacrylic acid and methanol. More particularly, it relates to an improvement of a method for esterifying methacrylic acid with methanol using an ion-exchange resin.
Known methods for producing methyl methacrylate from methacrylic acid and methanol use sulfuric acid or a strongly acidic ion-exchange resin as a catalyst.
The method using a strongly acidic ion-exchange resin is lower in methacrylic acid conversion than the method using sulfuric acid; however, the former is collectively considered to be more advantageous than the latter because the former requires no process for treating sulfuric acid incorporated into an oil layer and low-grade materials may be satisfactorily used as the reactor materials.
The production of methyl methacrylate using a strongly acidic ion-exchange resin has heretofore been carried out in a fixed bed reactor. However, an equilibrium exists in the esterification reaction, and hence, when the reaction is effected in a fixed bed reactor, it is impossible to achieve the high conversion of methacrylic acid at the outlet of the reactor. In order to shift the equilibrium to the product side, usually the mole ratio of methanol to methacrylic acid is increased (see Japanese Patent Kokoku No. 61-4,378). In this case, unreacted methacrylic acid and unreacted methanol are separated from the reaction mixture and thereafter recycled to the reactor (see Japanese Patent Kokoku No. 48-1,369).
In the conventional method using a fixed bed reactor, the mole ratio of methanol to methacrylic acid is 1.2-2.0, and the method is carried out with a methacrylic acid conversion of 30-90%. Therefore, there is a problem that it follows that a large excess of methanol is recovered by distillation and hence the energy consumption is great. There is another problem that since the impurities contained in the starting material and polisher of methacrylic acid are deposited, the reaction mixture does not flow uniformly on the whole of the ion-exchange resin layer, the catalyst activity is degraded owing to contamination of the ion-exchange resin surface, and the catalyst life becomes extremely short.
In order to solve the above problems, it can be considered that the fixed bed reactor be replaced with a stirring type reactor or a fluidized bed reactor in which the ion-exchange resin is always in the fluid state. However, this cannot always achieve the high conversion of methacrylic acid unlike the method using the fixed bed reactor, and when it is intended to remove water and methyl methacrylate, which are products, by distillation while conducting the reaction, methanol is preferentially distilled out to water because the boiling point of the azeotropic mixture of methyl methacrylate and methanol is lower than that of water and not only is it impossible to shift the equilibrium to the product side but also dehydration operation becomes impossible because the distillate does not separate into two liquid layers.